Xerographic charging system having means for providing an air cushion between the charging device and the xerographic drum



May 7, 1968 J. YOUNG ET AL 3,382,360 XEROGRAPHIC CHARGING SYSTEM HAVINGMEANS FOR PROVIDING AN AIR CUSHION BETWEEN THE CHARGING DEVICE AND THEXEROGRAPHIQDRUM Filed Sept. 10, 1965 5 Sheets-5heet 1 /I!Lllllli May 7,1968 YOUNG ET AL 3,382,360

XEROGRAPHIC CHARGING SYSTEM HAVING MEANS FOR PROVIDING AN AIR CUSHIONBETWEEN THE CHARGING DEVICE AND THE XEROGRAPHIC DRUM Filed Sept. 10,1965 5 Sheets-Sheet 2 COMPRESSED AIR POTENTIAL SOURCE ATTORNEYS May 7,1968 YQUNG ET AL 3,382,360

XEROGRAPHIC CHARGING SYSTEM HAVING MEANS FOR PROVIDING AN AIR CUSHIONBETWEEN THE CHARGING DEVICE AND THE XEROGRAPHIC DRUM Filed Sept. 10,1965 3 Sheets-Sheet 5 V V r v war-5mm 4 INVENTORS. JAMES E. YOUNG "RENTw. HEMPHILL BY RAYMONfiT WRIGHT W -&..M

A TTORN E Y5 United States Patent 3,382,360 XEROGRAPHIC CHARGING SYSTEMHAVING MEANS FOR PROVIDING AN AIR CUSHION BETWEEN THE CHARGING DEVICEAND THE XEROGRAPHIC DRUM James E. Young, Pittsford, Kent W. Hemphill,Rochester, and Raymond T. Wright, Penfield, N.Y., assignors to XeroxCorporation, Rochester, N.Y., a corporation of NewYork Filed Sept. 10,1965, Ser. No. 486,304

9 Claims. (Cl. 250-495) This invention relates to xerography and, moreparticularly, to an improved corona generating device for imposing auniform electrostatic charge on a xerographic plate.

In the process of xerography, for example, as disclosed in CarlsonPatent 2,297,691, a xerographic plate comprising a layer ofphotoconductive insulating material on a conductive backing is given auniform electric charge over its surface and is then exposed to thesubject matter to be reproduced, usually by conventional projectiontechniques. This process discharges the plate areas in accordance withthe radiation intensity that reaches them, and thereby creates anelectrostatic image on or in the photoconductive layer. Development ofthe latent image is effected with an electrostatically charged,finely-divided material, such as an electroscopic powder that is broughtinto contact with the photoconductive layer and is held thereonelectrostatically in a pattern corresponding to the electrostatic latentimage. Thereafter, the developed xerographic powder image is usuallytransferred to a support surface to which it may be fixed by anysuitable means.

In automatic equipment employing the principles of xerography, it iscommon to employ a xerographic plate in the form of a cylindrical drumwhich is continuously rotated through a cycle of sequential operationsincluding charging, exposure, developing and transfer. During the cycle,copy is reproduced onto a support surface and, after transfer, the plateis cleaned before reuse. It is usual to charge the xerographic platewith corona of positive polarity by means of a corona generating devicehaving an electrode which when supplied with potential above the coronathreshold produces an emission of corona ions that deposit onto theplate surface. Typical of the corona generating devices employedheretofore are those described in US. Patent 2,777,957 to Walkup and U5.Patent 2,836,725 to Vyverberg, each constructed generally of anelectrode wire or wires supported relatively close to the surface to becharged. In the former, a suppressing grid is placed intermediate theelectrode and the plate and maintained at a potential desired foruniformly charging the plate surface; while in the latter, a groundedmetallic shield generally surrounds the electrode except for an openingthrough which charge is emitted whereby control over the uniformity ofthe charge on the plate surface is effected by the relative movementthereof with respect to the electrode.

Inherent in xerographic apparatus of the type described above is thedifliculty in uniformly charging the surface of a xerographic drum thatmust be rotated at high speeds during prolonged, continuous operation.This lack of uniformity has been primarily attributed to non-uniformspacing between electrode and plate, variations in thickness andelectrical characteristics of the sensitized layer, and dirt whichaccumulates on the corona wires. Attempts to improve the uniformity ofcorona charging by the generating devices mentioned above, have requiredthe corona wires to be energized to a potential substantially in excessof the corona threshold potential thereby minimizing the effect of theforegoing in detracting from the 3,382,360 Patented May 7, 1968uniformity of charge on the xerographic drum. At times this has resultedin overcharging the photoconductive layer of the xerographic drum.Notwithstanding the application of higher potentials to the coronawires, it has not been possible to achieve a high degree of uniformityfor the charge on a xerographic drum prior to its exposure.

Now in accordance with the instant invention, there has been discovereda novel corona generating apparatus in which a moving air cushion isprovided for supporting an array of corona generating wires in floatingoverlying relationship with the surface to be charged. In this manner itis possible to pneumatically urge corona discharge onto the surfacepermitting substantially lower potential difference above the coronathreshold on the corona Wire per unit of time than has been requiredpreviously. At the same time, any differences in corona wire outputalong the wire length caused by dust accumulations or output changeseffected by ambient air conditions are obviated since the airflowdelivered cleans the wire and insures a relatively uniform airflowaround the wire. Moreover, the airflow passing between the coronagenerating device and chargeable surface spreads the charge uniformlyover the surface and helps clean the surface from dust accumulations orexcess developing material that has not been removed by the precedingcleaning step.

It is therefore an object of this invention to provide apparatus foruniformly charging a recipient surface moving relative thereto at highspeeds in an efficient manner.

Another object of this invention is to provide charging apparatuscapable of maintaining a substantially constant spacing between a coronagenerating wire and a recipient surface to be charged.

Still another object of this invention is to provide an improvedconstruction and design for a corona generating device to be used forcharging xerographic plates.

Still a further object of this invention is to provide xerographiccharging apparatus having self'contained means to continuously clean acorona discharge wire and surface to be charged while charge is beingapplied thereto.

For a better understanding of the invention, as well as other objectsand further features thereof, reference is had to the following detaileddescription of the invention to be read in connection with theaccompanying drawings, wherein:

FIGURE 1 is a schematic illustration of an automatic xerographicapparatus embodying the corona generator device of the invention;

FIGURE 2 is a perspective view of one embodiment of the coronagenerating device with parts of the device broken away to show detailstherein;

FIGURE 3 is a plan view of the bottom of the corona generating device ofFIGURE 2;

FIGURE 4 is a section view along lines 4-4 of FIG- URE 3; and

FIGURE 5 is a partial sectional view of still another embodiment of acorona generating device in accordance with the invention.

For a general understanding of the xerographic processing system inwhich the invention is incorporated, reference is had to FIGURE 1 inwhich the various system components are schematically illustrated. As inall xerographic systems based on the concept disclosed in theabove-cited Carlson patent, a radiation light image of copy to bereproduced is projected onto the sensitized surface of a xerographicplate to form electrostatic image thereon. Thereafter, the latent imageis usually developed with an oppositely charged developing material toform a xerographic powder image corresponding to the latent image on theplate surface. The powder image is then electrostatically transferred toa support surface to which it may be fused by any suitable form offusing device, whereby the powder image is caused permanently to adhereto the support surface.

The xerographic apparatus described herein typically may be of thetypedescribed in Cerasani et al., U.S. Patent 3,076,392. In theapparatus referred to, copies to be reproduced, which usually arepositive in form, are placed on a support tray from which they are fedonto a suitable transport mechanism designated 11. Suitable drive meansare provided for the transport mechanism from motor 12 to endless belts13 whereby the copy is moved past the optical axis of projection system14 and is illuminated by projection lamp LMP-l. The image of the copy isreflected by mirror 15 through an objective lens 16 and then isreflected by mirror 17 downwardly through a variable slit apertureassembly 18 and onto the surface of the xerographic plate in the form ofdrum 19.

Xerogra-phic drum 19 includes a cylindrical member mounted in suitablebearings in the frame of the machine and is driven in a clockwisedirection by a motor 24 at a constant rate that is proportional to thetransport rate of the copy, whereby the peripheral rate of the drumsurface is identical to the rate of movement of the reflected lightimage. The drum surface comprises a layer of photoconductive material25, such as vitreous selenium, on a conductive backing 26, such asaluminum. The photoconductive material is sensitized prior to exposureby means of a corona generating device 30 in accordance with theinvention, as will be described more fully below. Corona generatingdevice 30 is energized from a suitable high potential source 31 and hasconnected to it a compressed air source 32.

Exposure of the drum to the light image discharges the photoconductivelayer in the areas struck by light, whereby there remains on the drum alatent electrostatic image in image configuration corresponding to thelight image projected from the copy. As the drum surface continues itsmovement, the electrostatic latent image passes through a developingstation 35 at which a two-component developing material 36, which may beof the type disclosed in Walkup Patent 2,638,416, is cascaded over thedrum surface by means of developing apparatus 37. In

the developing apparatus, developing material is carried upward byconveyor 39 driven by suitable drive means from motor 40 and is releasedonto chute 41 wherefrom it is cascaded down over the drum surface. Tonercomponent 42 of the developer, which is consumed in developing, isstored in dispenser 43 and is released in amounts controlled by gate 44.

After developing, the xerographic powder image passes a dischargestation 50 at which the drum surface is illuminated by a lamp LMP-Z,whereby residual charges on the non-image areas of the drum surface arecompletely discharged. Thereafter, the powder image passes through animage transfer station 51 at which the powder image is electrostaticallytransferred to a support surface web 52 by means of a second coronagenerating device 53 similar to corona generator 30 as will behereinafter described. 1 1

The support surface to which the powder image is transferred may be ofany convenient type, such as paper, and is obtained from a supply roll54 to pass over guide rolls 55 and 56 and over suitable tensioning rollsbeing directed into surface contact with the drum in the immediatevicinity of transfer corona generating device 53. After transfer, thesupport surface is separated from the drum and is guided through asuitable heat fusing apparatus 57 whereby the powder image ispermanently affixed to the support surface. Thereafter, the supportsurface is fed over a further system of guide and tensioning rolls andonto a take-up roll 58 that is driven by motor 59.

Following separation of the support surface from the drum, a coronagenerating device 65, likewise constructed in accordance with theinvention hereof, as will be further described below, directs negativeelectrostatic charge to the residual powder image on the drum surface.The drum surface then passes through a cleaning station 66 at which thesurface is brushed by cleaning brushes 67, rotated by a motor 68,whereby residual developing material remaining on the drum is removed.Suitable light traps are provided in the system to prevent any lightrays from reaching the drum surface, other than the projected image,during the period of drum travel immediately prior to sensitization bycorona generating device 30 until after the drum surface is completelypassed through the developing station 35.

Since the construction of all the foregoing charging units is similar, adescription of charging tmit 30 is deemed sufficient. Referring nowspecifically to FIGURES 24, there is shown corona generating device 30constructed in accordance with a first embodiment of the presentinvention. The unit generally comprises a housing consisting of top wall76 and sidewalls 77 made from any suitable electrically conductivematerial that is connected to ground. Formed in top 76 is an opening 78to which is connected compressed air source 32 through a conduit 82provided with a valve 84. Connecting walls 77 of housing 75 andcoextensive therewith is an elec-' trically insulating frame 92 forminga plenum chamber 93 within the housing. There is embedded in frame 92 anarray of corona discharge wires 95 to which is connected a potentialsource 31 ranging from about 3,000 to 10,000 volts D.C. Frame 92 andwire array 95 are arranged to lie in a plane generally parallel to thesurface of advancing xerographic drum 19 and are supported above thesurface of the drum on a cushion of air in a manner which will becomeapparent.

Extending from sidewalls 77 are pins 101 which are received through anelongated opening 103 in supporting brackets 105. Coincident with eachopening 103 is a spring 107 for urging housing 75 toward the surface ofdrum 19 against the cushion of air which is desirably able to overcomethe natural stiffness of the spring. A screw 109 is provided foradjusting the bias on spring 107. As can readily be appreciated,elongated openings 103 in supporting brackets allow housing 75 to bedisplaced vertically, but prevent lateral movement thereof with re- 5spect to drum 19. Also, the brackets serve as means for maintaining thecharging unit adjacent the surface of the drum especially where theforce of gravity on the unit is away from the drum center as in the caseof charging units 53 and 65. The number of biasing connections used willdepend upon the size and shape of the unit and those shown are merelyillustrative.

As best shown in FIGURES 3 and 4, frame 92 has a plurality of orifices116 which are in registry with a plurality of larger slots 118 recessedin the bottom above wire array 95 so that communication is providedbetween plenum chamber 93 and the exterior of charging unit 30. Slots118 are elongated in a direction transverse to the path in whichxerographic drum 19 is advanced past the charging unit, thereby exposingthe drum surface to the corona current generated from wire array 95. Airfrom plenum chamber 103 is directed first through restricted orifices116 before reaching larger slots 118 so that air at sufficient pressureis generated to lift charging unit 30 onto an air cushion formed betweenframe 92 and the about 10- to 20 pounds p.s.i.g. to float a unitweighing about a pound. In case of a loss in air pressure, a sensorswitch 121 actuates two or more solenoids 123. which push upwardlyagainst extensions 125 of housing 75 thereby preventing the chargingunit from'contacting drum 19 during charging. Sensor switch 121 may belocated in any convenient place, such as inside the air plenum.

Frame 92 is made from a light weight are resistant material that maysuitably be formed of plastic in an injection mold wherein wire array 95is placed prior to injecting the plastic material into the mold. Typicalplastic materials suitable for this purpose are dialphthalate, epoxy andRexolite, an acrylic plastic. Wire array 95 is a conductive copper thatmay have any suitable geometric arrangement, such as parallel wiresconnected at their ends, as best shown in FIGURE 3. Alternatively, thearray may consist of a wire mesh. It has been found that wire diametersranging from 2 to 5 mils work well as a corona discharge wire when usedwith potentials ranging from 3,000 to 10,000 volts D.C. Any suitablemeans may be used for uniting housing 75 with insulating frame 92, suchas an adhesive.

In operation, a continuously moving air cushion is formed between theunderside of the charging unit 30 and xerographic drum 19 supporting thecharging unit in overlying floating relationship relative to the surfaceof the drum. The spacing between drum 19 and the bottom of the chargingunit may be controlled by either regulating the pressure of plenumchamber 103 by valve 84 or adjusting the biasing on springs 107 by meansof screws 109, or both. Not only does the air float the unit but it hasbeen found that fluid flow across frame 92 also blows the coronacharging current of ions or electrons, depending upon the polarity ofcharge, over the surface of drum 19, thereby more uniformly distributingthe charge thereon. It is believed the airstream serves in this respectto accelerate the ionizing collisions that occur outwardly from wirearray 95 to intensify the charging current. Because of this elfect, thenumber of ions or electrons available for charging dr-um 19 is believedincreased many times due to the sweeping action of the positive airflowacross the electric field surrounding wire array 95. Any dust thatcollects on either the framework 95 or the surface of xerographic drum19 is blown free and clear by the fluid pressure. All of the foregoingenables alower potential to be applied for generating corona current ata given threshold potential.

In another embodiment of the present invention there is included asuppressor grid 152 which is spaced intermediate the array of coronadischarge wires 155 and xerogra-phic drum 159, as shown in FIGURE 5.Grid 152 is in the form of a conductive screen having a potentialsubstantially the same as the potential desired on the drum so thations, in the case of positive charging, pass between the grid andcontinue onto the drum as long as a field exists between the grid anddrum. Suitable potentials for wire array 155 and grid 152 are 3,000volts and 400 volts, respectively.

As in the case of the first embodiment shown in FIG- URES 2 through 4,the charging unit contains a plenum chamber 161 which is formed by ahousing 163 and a frame 165. Frame 165 is a composite structure made upof a conductive plate 171 containing orifices 172 and an insulatingmember 175 having larger openings 176. Orifices 172 and openings 176 arein registry so that comrnunication is provided between plenum chamber161 and the exterior of the charging unit to form an air cushion forfloating the charging unit in overlying relation to drum 159. Sinceweight is an important consideration, housing 163, as well as insulatingmember 175, which contains grid 152 and wire array 155, are desirablyconstructed from any suitable light weight dielectric material, such asplastic.

In accordance with the invention, fluid pressure from plenum chamber 161is directed downwardly and across frame 165, as shown by the arrows inFIGURE 5, blowing the charging current uniformly over the surface ofxerographic drum 159. Because of the close proximity of grounded plate171 to corona wire array 155, a strong electrostatic field is formedtherebetween enhancing the ion flow toward xerographic drum 159. Also,the potential applied to grid 152 permits greater selectivity of thecharge desired on drum 159. In this manner, higher charge efficiency anduniformity are maintained at a given threshold potential. As alreadymentioned, an air cushion is formed between the charging unit andxerographic drum 159 for supporting the charging unit at a fixeddistance from the surface of the drum and for serving as aself-contained cleaning medium during charging.

By the above description there has been disclosed a corona generatingdevice capable of charging a xerographic surface moving relative theretouniformly and efficiently. Heretofore, corona charging could beaccomplished satisfactorily only by imposing potentials on a corona wireor wires that greatly exceeded the threshold potential. With thearrangement described, the charging unit has been found to disseminatemuch higher charging currents to the xerographic surface per unit timeat a given threshold potential allowing lower potentials to be used. Atthe same time, the positive airflow between the floating charging unitand the xerographic surface provides a uniform charge across the surfaceas well as a continuous cleaning medium. Hence, not only does increasedcorona current enable reducing the potential applied to the corona wire,but the uniformity of charge on the xerographic surface atfords higherquality reproduction.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication i intended to cover such such modifications as will becomeapparent to those skilled in the art upon a reading of this disclosure;these are intended to be encompassed within the scope of this invention.

What is claimed is:

1. A corona generating device for uniformly charging a recipient surfacemoving relative thereto comprising:

(a) contiguous walls partially forming an enclosure, including an inletopening in one of said walls for connecting a source of compressed airthereto;

(b) a frame coextensive with and connected to said walls to form aplenum chamber therebetween, said frame containing a plurality of airpassages communicating therethrough between said plenum chamber to theexterior of said frame;

(c) an array of corona discharge wires supported in said frame at leastpartially exposed to the exterior thereof;

(d) a source of potential connected to said wires sufiicient to generatea corona charging current thereon; and

(e) means for supplying compressed air to said opening to effect duringcharging an air flow from said plenum through said passages providingsupport between said frame and the surface to be charged.

2. Apparatus according to claim 1 wherein said means for supplyingcompressed air to said opening includes a pressure control valve formaintaining a delivered pressure suflicient to control a desired spacingduring charging between said frame and the surface to be charged.

3. Apparatus according to claim 1 including elastic means opposing saidair cushion to urge said device toward the surface to be charged, thenatural stiffness of said elastic means being less than the force ofsaid air cushion.

4. Apparatus according to claim 1 including means for preventing saiddevice from contacting a surface being charged when air pressure fromsaid supply means is less than a predetermined minimum.

5. Apparatus according to claim 1 including a suppressor grid supportedin said frame in the corona discharge path from said array of coronawires, said grid being connected to a potential substantially the sameas that to be applied to a surface to be charged.

6. Apparatus according to claim 1 where in said frame consists of adielectric module.

7. Apparatus according to claim 1 wherein said frame comprises adielectric member having slots through which said array is partiallyexposed and a conductive plate containing apertures in registration withsaid slots.

8. In a xerographic apparatus including a xerographic drum comprising aphotoconductive layer on a conductive substrate, and means tocontinuously rotate said drum sequentially past a plurality ofprocessing stations, a corona generating device for applying a uniformelectrostatic charge onto the surface of said drum comprising incombination:

(a) contiguous walls partially forming an enclosure, including an inletopening in one of said walls for connecting a source of air thereto;

(b) a frame coextensive with and connected to said walls to form aplenum chamber therebetween being positioned generally parallel to thephotoconductive layer, said frame containing a plurality of air passagescommunicting therethrough between said plenum chamber to the exterior ofsaid frame;

(0) means for selectively urging said device against said air cushion;

(d) an array of corona discharge wires supported in said frame at leastpartially exposed to the exterior thereof;

(e) a source of potential connected to said wires sufficient to generatea corona charging current thereon; and

(f) means for supplying compressed air to said opening to effect duringcharging an air flow from said plenum through said passages providing anair cushion support between said frame and said xerographic drum.

9. A method for depositing electrical charge from an electrode unit ontoa relatively moving insulating surface comprising the steps of:

(a) applying an air flow between the electrode unit and the insulatorsurface of pressure sufficient to float the unit relative thereto; and

(b) applying a charge generating potential to said electrode unit toimpart charge therefrom onto said surface.

No references cited.

RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner.

8. IN A XEROGRAPHIC APPARATUS INCLUDING A XEROGRAPHIC DRUM COMPRISING APHOTOCONDUCTIVE LAYER ON A CONDUCTIVE SUBSTRATE, AND MEANS TOCONTINUOUSLY ROTATE SAID DRUM SEQUENTIALLY PAST A PLURALITY OFPROCESSING STATIONS, A CORONA GENERATING DEVICE FOR APPLYING A UNIFORMELECTROSTATIC CHARGE ONTO THE SURFACE OF SAID DRUM COMPRISING INCOMBINATION: (A) CONTIGUOUS WALLS PARTIALLY FORMING AN ENCLOSURE,INCLUDING AN INLET OPENING IN ONE OF SAID WALLS FOR CONNECTING A SOURCEOF AIR THERETO; (B) A FRAME COEXTENSIVE WITH AND CONNECTED TO SAID WALLSTO FORM A PLENUM CHAMBER THEREBETWEEN BEING POSITIONED GENERALLYPARALLEL TO THE PHOTOCONDUCTIVE LAYER, SAID FRAME CONTAINING A PLURALITYOF AIR PASSAGES COMMUNICTING THERETHROUGH BETWEEN SAID PLENUM CHAMBER TOTHE EXTERIOR OF SAID FRAME; (C) MEANS FOR SELECTIVELY URGING SAID DEVICEAGAINST SAID AIR CUSHION; (D) AN ARRAY OF CORONA DISCHARGE WIRESSUPPORTED IN SAID FRAME AT LEAST PARTIALLY EXPOSED TO THE EXTERIORTHEREOF; (E) A SOURCE OF POTENTIAL CONNECTED TO SAID WIRES SUFFICIENT TOGENERATE A CORONA CHARGING CURRENT THEREON; AND (F) MEANS FOR SUPPLYINGCOMPRESSED AIR TO SAID OPENING TO EFFECT DURING CHARGING AN AIR FLOWFROM SAID PLENUM THROUGH SAID PASSAGES PROVIDING AN AIR CUSHION SUPPORTBETWEEN SAID FRAME AND SAID XEROGRAPHIC DRUM.